The present invention relates to the use of bismuth-compounds in extreme pressure grease lubricant compositions for application in rolling bearings.
The invention further relates to a bismuth-additive containing, polymer-thickened grease composition for use in rolling bearings.
Generally, in cases wherein rolling bearings operate under extreme loading conditions and with long service intervals, extreme pressure grease lubricants are applied in order to make sure that an appropriate amount of lubricant film is always available within the bearing, in particular on the raceways and rolling elements thereof.
Such extreme pressure lubricant compositions generally contain an oil, a soap thickener, one or more EP additives and optionally further additives. The EP additives form a friction-reducing film on the metal surfaces of the bearing, usually due to a chemical reaction of the additives with the surface metals. The function of the supplied lubricant extreme-pressure additives is to minimize wear and to prevent scuffing and welding between contacting surfaces. As such, lead/sulfur-containing additives can be used.
However, these lead additives are not acceptable because of their toxicity and from an environmental viewpoint. Therefore, the lead/sulfur-containing additives are now usually replaced by sulfur/phosphorous EP additives. However, it has now been found that these sulfur/phosphorous additives reduce the service life of the bearings.
Extreme pressure lubricant compositions also contain a soap thickener, such as lithium 12-hydroxy stearate, which provides the grease with the desired physical and chemical structure. The grease should be able to maintain this structure in the bearing as long as possible under high temperature, vibratory and mechanical shearing action.
In this respect, it is necessary to maintain the mechanical stability of the soap or thickener of the grease during extended periods. As long as this soap structure can be maintained, the grease is able to hold in place the oil component which can regularly provide the lubricating properties.
In cases where the soap structure is damaged the grease is no longer able to hold the oil in place, which will then drain away out of the bearing. As a consequence the lubricating properties of the grease are lost and the bearing service life is reduced considerably if the grease is not replenished at short intervals. Furthermore, the grease should be chemically non-aggressive with respect to the metal parts of the bearing (contacts), in particular with the above environment with high temperatures and vibration.
In these respects the lead/sulfur- and sulfur/phosphorous EP additives containing grease lubricant compositions according to the state of the art are not satisfactory. The aim of the invention is therefore to obviate these disadvantages.
Ransom, U.S. Pat. No. 5,385,683, discloses a lubricant composition, moving metal parts, a method for reducing friction, as well as a method for preparing a composition comprising an oil, a bismuth containing compound and other additives. However, Ransom relates to lubricating oils, not lubricating greases.
Also, Ransom describes the use of a combination of a bismuth additive and a tin additive, which react to form a bismuth/tin alloy, which coates the bearing surfaces. According to the present application, no tin additive is present, and such an alloy cannot be formed.
Furthermore, Ransom is directed to reducing friction and wear in sliding metal contacts, i.e. metals moving with respect to one another. Ransom does not relate to the specific problems of roller bearings: the Timken bearing test used in the Examples measures wear in sliding contacts, as discussed below.
U.S. Pat. No. 5,266,225 (Hall) describes a number of bismuth compounds (soaps) for use as EP additives, leading to reduction of wear and improved lubricity in moving metal parts, such as pistons, piston rings etc. of automotive engines.
Hall does not relate to roller bearing applications, nor is any beneficial effect on the useful service life of roller bearings mentioned or suggested. For instance, in example 7, Hall uses the standard Timken-test, which measures wear in sliding contacts.
Also, Hall mainly relates to bearings operated under sandy conditions whereby the bismuth compounds provide a protective film on the sand and grits which prevents wear.
U.S. Pat. No. 3,028,334 (Wilson) describes the use of gamma-ray absorbing compounds in a preparation of lubricants resistant to atomic radiation, for which purpose no specific preference for bismuth over for instance titanium, molybdenum, mercury or lead is given. Also, the use of bismuth compounds for improving the useful service life of greased roller bearings is neither described nor suggested.
A grease containing a bismuth additive is also known from NLGI SPOKESMAN, Vol. 57, Nr. 2, May 1993, O. ROHR "Bismuth, a new metallic but non-toxic replacement for lead as EP additive in greases", pages 6.50-13.57. In this article, it is described that a bismuth additive promotes the formation of a film on the rolling bearing metal surfaces and therefore could serve as a replacement for lead as extreme pressure additive in grease. The bismuth additive indeed appears to offer even better lubricating properties than a lead additive, in particular under high load, high temperature and high sliding speed conditions. Also it is mentioned that the organobismuth compound functions as a corrosion inhibitor and as an antioxidant.
However, this article is silent with respect to the field of the present invention, that is the provision of an extended useful service life of the greases and thus the bearing (fatigue) life. Whereas it is reported that the bismuth additive beneficially influences the lubricating properties of the oil component of the grease, no reference is made to any favourable effects on bearing service life.
Also, both the Timken test and the Shell four ball test used by Rohr measure friction and wear during brief periods of time, i.e. until the "welding point" is reached where the bearing surfaces melt together under pressure and frictional heat.
The application of bismuth additives in a lubricant is also addressed in SU-A-1384603 by Egorenkov et al. Here, the bismuth is added to lubricating oil compounds for sliding contact surfaces, i.e. "for metal and metal-polymer friction pairs".
Although it is stated that the bismuth additives reduce the friction in a sliding contact, no reference is made to roller element bearings or to achieving an extended service life thereof. i.e for instance as a result of improved lubricant life or improved fatigue life. Furthermore, Egorenkov teaches bismuth additives only in combination with cadmium-additives, such as a combination of cadmium oleate or stearate and bismuth oleate or stearate.
In general, the above prior art teaches the use of bismuth compounds for reducing friction and wear, i.e. abrasive and/or adhesive wear. This is generally associated with sliding contacts, i.e. metal surfaces moving with respect to each other.
The present invention is particularly concerned with rolling contacts in roller/rolling bearings. In such ball-type bearings, sliding contacts are minimal and friction and wear are generally low under well lubricated conditions, which is also independant of the type of grease lubricant used. Therefore, in rolling element bearings, under conditions of high bearing temperatures and high contact stress at normal speed, or under conditions of high temperatures and high speed at low load, friction and wear are not of concern, even with prior art sulfur/phosphorous or sulfur/lead additives.
However, in roller bearings under the abovementioned conditions, after at least tens to more than a hunderd hours of continous operation, stress corrosion, fatigue and pitting of the bearing surfaces can become a problem, as well as reduction of grease life; these are some of the main problems addressed by the present invention.
In comparison, friction and wear are problems which occur immediately once the bearing is put into operation. Generally, if friction and wear due to sliding contacts were to be a problem in rolling element bearings, such bearings would fail immediately (i.e within hours) because the heat of friction would destroy the bearing, i.e. by fusing the bearing surfaces.
However, because there is little sliding in ball bearing contacts, especially between the raceway and the ball, after long periods of operation, other kinds of mechanisms for inducing bearing failure become relevant, in particular fatigue and pitting, as well as grease failure. (In sliding contacts, friction and wear would destroy the bearing long before the grease fails.)
Therefore, fatigue phenomena, such as stress corrosion induced fatigue in roller bearings, which are reduced according to the invention, are not related to the problems of friction and wear in sliding contacts addressed by the above art. This is also evident from the fact that tests for measuring friction and wear used in said art, such as ASTM D-1743 and the Shell-4-ball test (ASTM D-2596) mentioned by Rohr, as well as the Timken test described by Ransom and Hall, are suitable only for measuring friction and wear in sliding contacts during brief periods of time (i.e. 1-10 hours before the bearing fails); such tests cannot be used for measuring service life or bearing/grease failure in the context of the present invention, for which tests lasting tens or hunderds of hours are required, such as the "Deep groove ball bearing (DGBB 6206 2RS1)" test or the "taper roller bearing (580/572)" test described in the Examples hereinbelow.
For instance, the Timken bearing test used in the Examples of Ransom measures wear, i.e. at very low speeds and at very low temperatures (290 rpm and 150.degree. F.=65.degree. C., vide the tables). Also, said Timken-test is run over a period of maximum 240 minutes (vide the figures), which is insufficient to measure the grease failure and/or fatigue failure effects the present invention tries to overcome, which take hunderds of hours to develop.
Also, both the ASTM-D-1743 test and the Shell four ball test used by Rohr measure friction and wear during brief periods of time, i.e. until the "welding point" is reached where the bearing surfaces melt together under pressure and frictional heat.
Also, although Rohr measures "corrosion" in Table 3, this is measured in a standard EMCOR test, which measures water corrosion, not stress corrosion under the operating conditions of the invention as set out above. Also, the known "free"-sulfur containing greases give good corrosion protection in the Emcor test, whereas a free sulfur content is detrimental under the conditions of the invention, as further discussed hereinbelow.
Therefore, the present invention particularly relates to improved service life and grease life in a roller element bearing under conditions of high bearing (outer ring) temperatures (e.g. 80-130.degree. C.) and high contact stress (e.g. C/P=2-15) and normal bearing rating speeds; or under conditions of high speeds (e.g. NDM: 700,000 to 1.5 millions) and low load (e.g. C/P&gt;15) and high temperature (e.g. 80-130.degree. C.), and during long periods of operation (e.g. more than 50, preferably more than 100, more preferably more than 200 hours).
At these conditions of high temperature and high pressure, during long continuous operation, the chemical reactivity of the additives in the grease become very important. "Free" sulfur containing of releasing additives, such as the known lead/sulfur-or sulfur/phosphorous EP additives. will attack both the bearing surfaces, thereby promoting fatigue, such as induced by stress corrosion phenomena, as well as the grease structure itself (i.e. the thickener), leading to reduced grease life, reduced oxidation life and reduced mechanical/shear stability.
According to the present invention, stress corrosion and fatigue of the bearing surfaces is reduced, also, oxidation life and shear stability at high temperatures are improved, both factors extending the service life of the grease and the bearing. These factors affecting the bearing life are not a problem in sliding contacts, as the bearing surfaces will be destroyed by friction and wear long before the grease structure fails.